1. Field of the Invention
The present invention generally relates to interposers, and more particularly relates to an interposer particularly suited to highly miniaturized applications, a method for making the interposer, and an interposer made by a method.
2. Background Information
Interposers are devices used with electrical components to provide an electrical pathway from one electrical component to another. They are typically about the size of a postage stamp, and contain arrays of small contacts (each contact being one interposer) which are placed over one electrical component, so that a second electrical component can be placed in contact with the interposer for full electrical contact between the two components. Interposers may be used in a testing situation in which numerous electrical components are to be connected to a testing device. Interposers can also be used in a situation where an electrical component is to be interchangeable or replaceable, in order to make replacement easier.
One particular type of contact that needs to be contacted by interposers is a solder ball. This is a generally semi-spherical shaped ball, which forms the electrode of one electrical component, such as a silicon chip. The electrical connections of an interposer need to be able to contact the solder ball, and possibly correct centering problems and also form a good electrical connection in spite of the possibility of a layer of oxidation being present on the solder ball. As solder balls on electrical components have become increasingly smaller, the present technology to make interposers that make a clean and efficient connection to such solder balls has been surpassed. Several ways are utilized to try to make a good connection with solder balls. These include a metal pinch contact, which is like the tips of a very small pair of tweezers fitting around the solder ball. Another variation on this approach is a connection in which the tips that fit around the solder ball are generally shaped like small spoons, and fit the contours of the sides of the solder ball. Another approach to this problem is the metal “y” contact. Still another interposer design to contact the solder ball is by using a contact of elastomeric material with metallic beads suspended in the material, which form a rough surface for contacting the ball. A similar approach is to use a conductive polymer bump mounted on ceramic. Another approach is to form a pocket etched in silicon. Still another approach is to use a metal probe that contacts the solder ball. Such a metal probe can have a pocket into which part of the solder ball can fit.
These approaches have proven to be only partially successful. A problem with them is that the smallest scale that these devices can be built is a scale in which the center of the balls is approximately 0.75 millimetres (mm) apart. This is called a 0.75 mm pitch array. In such an array, the solder balls are approximately 0.35 mm in diametre. As solder balls become smaller, a 0.65 mm pitch array and even smaller pitch arrays are preferable, and an interposer must be designed that can interface with such a pitch array. Current technologies are unable to achieve a good connection with a pitch array of less than 0.75 mm.
One approach to achieving a good connection between the contact pad of an interposer and a solder ball is to have the contact pad include a hollow space, or well, in the center. This well is achieved in the current technology by drilling a hole through the interposer. This results in an interposer with a through hole. On the side of the interposer opposite from the side that contacts the solder ball, an elastomeric pad is often attached to the lower contact surface. The elastomeric material is filled with metallic balls that create an electrical connection. The ability of the elastomeric pad to flex slightly allows the lower surface of the interposer to compensate for co-planarity problems of a chip or testing machine. The problem with this type of an interposer is that the elastomeric material is inevitably subjected to heat. When a chip is being tested, it is tested in an environment of approximately 125 degrees Centigrade. When a chip is under this temperature, its electrical circuitry is tested. Thus the testing equipment must be able to also survive that temperature. In this temperature range, volatile elements of the elastomeric material tend to off gas. As the elastomeric material emits these volatile gases, an interposer with a through hole will allow these off gases to pass through the interposer and contact the solder balls. They can be deposited on the solder ball, and result in poor electrical connections when the chip is used in its final application. Thus, a hole through the interposer is undesirable, but a well on the surface of the interposer that contacts the solder ball is desirable. An elastomeric pad is also desirable.
Therefore, it is an object of the present invention to make an interposer that is suitable for use with the new generation of smaller pitched solder balls. It is a further object of the invention to provide a method for making an interposer that will result in an interposer that operates with small pitched solder ball arrays. It is a further object of the invention to provide an interposer made by a method in which vias are cut with a laser in the insulating layer. The invention also yields an interposer with a contact well, which does not go all the way through, and which has a layer that blocks off gases.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.